Nutrient Economy Research Articles

Convergent responses of nitrogen and phosphorus resorption to nitrogen inputs in a semiarid grassland

Human activities have significantly altered nitrogen (N) availability in most terrestrial ecosystems, with consequences for community composition and ecosystem functioning. Although studies of how changes in N availability affect biodiversity and community composition are relatively common, much less remains known about the effects of N inputs on the coupled biogeochemical cycling of N and phosphorus (P), and still fewer data exist regarding how increased N inputs affect the internal cycling of these two elements in plants. Nutrient resorption is an important driver of plant nutrient economies and of the quality of litter plants produce. Accordingly, resorption patterns have marked ecological implications for plant population and community fitness, as well as for ecosystem nutrient cycling. In a semiarid grassland in northern China, we studied the effects of a wide range of N inputs on foliar nutrient resorption of two dominant grasses, Leymus chinensis and Stipa grandis. After 4years of treatments, N and P availability in soil and N and P concentrations in green and senesced grass leaves increased with increasing rates of N addition. Foliar N and P resorption significantly decreased along the N addition gradient, implying a resorption-mediated, positive plant-soil feedback induced by N inputs. Furthermore, N:P resorption ratios were negatively correlated with the rates of N addition, indicating the sensitivity of plant N and P stoichiometry to N inputs. Taken together, the results demonstrate that N additions accelerate ecosystem uptake and turnover of both N and P in the temperate steppe and that N and P cycles are coupled in dynamic ways. The convergence of N and P resorption in response to N inputs emphasizes the importance of nutrient resorption as a pathway by which plants and ecosystems adjust in the face of increasing N availability.

The mycorrhizal‐associated nutrient economy: a new framework for predicting carbon–nutrient couplings in temperate forests

Understanding the context dependence of ecosystem responses to global changes requires the development of new conceptual frameworks. Here we propose a framework for considering how tree species and their mycorrhizal associates differentially couple carbon (C) and nutrient cycles in temperate forests. Given that tree species predominantly associate with a single type of mycorrhizal fungi (arbuscular mycorrhizal (AM) fungi or ectomycorrhizal (ECM) fungi), and that the two types of fungi differ in their modes of nutrient acquisition, we hypothesize that the abundance of AM and ECM trees in a plot, stand, or region may provide an integrated index of biogeochemical transformations relevant to C cycling and nutrient retention. First, we describe how forest plots dominated by AM tree species have nutrient economies that differ in their C-nutrient couplings from those in plots dominated by ECM trees. Secondly, we demonstrate how the relative abundance of AM and ECM trees can be used to estimate nutrient dynamics across the landscape. Finally, we describe how our framework can be used to generate testable hypotheses about forest responses to global change factors, and how these dynamics can be used to develop better representations of plant-soil feedbacks and nutrient constraints on productivity in ecosystem and earth system models.

Foliar mineral nutrient uptake in carnivorous plants: what do we know and what should we know?

Carnivorous plants (CPs) usually growin nutrient-poor, wet or aquatic environ-ments and possess foliar traps which cap-ture animal prey (Juniper et al., 1989).There are about 600 terrestrial and 50aquatic or amphibious species of CPswhich supplement the conventional min-eral nutrient uptake by roots or shootsfrom their environment by the absorp-tion of nutrients (mainly N, P, K, Mg)from prey carcasses captured by theirtraps (for the review, see Adamec, 1997,2002, 2011a). Among vascular plants, theyprobably have the greatest capacity offoliar mineral nutrient uptake which cancover 5–100% of their seasonal N and Pgain (consumption) but only 1–16% forK from captured prey (Adamec, 1997,2011a). The main ecophysiological strat-egy of terrestrial species as S-strategistsis slow growth and very effective min-eral nutrient economy. Due to new dis-coveries (e.g., Spomer, 1999; Andersonand Midgley, 2003; Pavloviˇc, 2012 ), theboundary between carnivory and non-carnivory remains slightly blurred. Inline with recent findings, the concept ofplant carnivory should be defined withan emphasis on the main benefit ofcarnivory—the uptake of mineral nutri-ents from prey(directly or indirectly) cap-tured bytraps.Moreover,asallplantswithglandularhairsarepotentially carnivorous(Spomer, 1999), a defining statement thatfoliar nutrient uptake from prey must be“ecologically significant” for CPs seemsreasonable (Plachno et al., 2009)whereascriteriasuchasproducingtheirowndiges-tive enzymes or prey attraction are onlymarginal.

Evolution of fruit traits in ficus subgenus Sycomorus (Moraceae): to what extent do frugivores determine seed dispersal mode?

Fig trees are a ubiquitous component of tropical rain forests and exhibit an enormous diversity of ecologies. Focusing on Ficus subgenus Sycomorus, a phenotypically diverse and ecologically important Old World lineage, we examined the evolution of fruit traits using a molecular phylogeny constructed using 5 kilobases of DNA sequence data from 63 species (50% of global diversity). In particular, we ask whether patterns of trait correlations are consistent with dispersal agents as the primary selective force shaping morphological diversity or if other ecological factors may provide a better explanation? Fig colour, size and placement (axial, cauliflorous, or geocarpic) were all highly evolutionarily liable, and the same fruit traits have evolved in different biogeographic regions with substantially different dispersal agents. After controlling for phylogenetic autocorrelation, we found that fig colour and size were significantly associated with fig placement and plant-life history traits (maximum plant height and leaf area, respectively). However, contrary to prevailing assumptions, fig placement correlated poorly with known dispersal agents and appears more likely determined by other factors, such as flowering phenology, nutrient economy, and habitat preference. Thus, plant life-history, both directly and through its influence on fig placement, appears to have played a prominent role in determining fruit traits in these figs.

Nitrogen and phosphorus economy of a legume tree-cereal intercropping system under controlled conditions

Considerable amounts of nitrogen (N) and phosphorus (P) fertilizers have been mis-used in agroecosystems, with profound alteration to the biogeochemical cycles of these two major nutrients. To reduce excess fertilizer use, plant-mediated nutrient supply through N2-fixation, transfer of fixed N and mobilization of soil P may be important processes for the nutrient economy of low-input tree-based intercropping systems. In this study, we quantified plant performance, P acquisition and belowground N transfer from the N2-fixing tree to the cereal crop under varying root contact intensity and P supplies. We cultivated Acacia senegal var senegal in pot-culture containing 90% sand and 10% vermiculite under 3 levels of exponentially supplied P. Acacia plants were then intercropped with durum wheat (Triticum turgidum durum) in the same pots with variable levels of adsorbed P or transplanted and intercropped with durum wheat in rhizoboxes excluding direct root contact on P-poor red Mediterranean soils. In pot-culture, wheat biomass and P content increased in relation to the P gradient. Strong isotopic evidence of belowground N transfer, based on the isotopic signature (δ15N) of tree foliage and wheat shoots, was systematically found under high P in pot-culture, with an average N transfer value of 14.0% of wheat total N after 21days of contact between the two species. In the rhizoboxes, we observed limitations on growth and P uptake of intercropped wheat due to competitive effects on soil resources and minimal evidence of belowground N transfer of N from acacia to wheat. In this intercrop, specifically in pot-culture, facilitation for N transfer from the legume tree to the crop showed to be effective especially when crop N uptake was increased (or stimulated) as occurred under high P conditions and when competition was low. Understanding these processes is important to the nutrient economy and appropriate management of legume-based agroforestry systems.

Crop Breeding for Low Input Agriculture: A Sustainable Response to Feed a Growing World Population

World population is projected to reach its maximum (~10 billion people) by the year 2050. This 45% increase of the current world population (approaching seven billion people) will boost the demand for food and raw materials. However, we live in a historical moment when supply of phosphate, water, and oil are at their peaks. Modern agriculture is fundamentally based on varieties bred for high performance under high input systems (fertilizers, water, oil, pesticides), which generally do not perform well under low-input situations. We propose a shift of research goals and plant breeding objectives from high-performance agriculture at high-energy input to those with an improved rationalization between yield and energy input. Crop breeding programs that are more focused on nutrient economy and local environmental fitness will help reduce energy demands for crop production while still providing adequate amounts of high quality food as global resources decline and population is projected to increase.

Plasticity in leaf and stem nutrient resorption proficiency potentially reinforces plant–soil feedbacks and microscale heterogeneity in a semi‐arid grassland

Summary1. The potential resorption of substantial amounts of nutrients from all vegetative organs of plants has large implications for the plant nutrient economy and for biogeochemical cycles. So far, most studies have focused on leaf nutrient resorption only. Besides, while evidence is growing that soil fertility changes impact on leaf nutrient resorption at a large spatial scale, hardly anything is known of such coupling at a small spatial scale.2. Here we show that nitrogen (N) in culms of four dominant grasses of northern Chinese steppes contributed from 17% to 36% to the total pool of N resorbed from above‐ground senescing parts and accounted for 25–52% of above‐ground litter N. These results demonstrate the tremendous importance of non‐leaf organs for plant nutrient economy and ecosystem nutrient cycling.3. More importantly, we found that even microscale variations in resource availability (soil inorganic N; soil moisture) can strongly impact on both leaf and culm N resorption proficiencies (RP) and absolute leaf N resorption of grasses. Moreover, plasticity was responsible for 86% and 43% of within‐site variance in leaf and culm RP, respectively, with the remainder owing to interspecific differences between the four grasses. These results imply a much larger role of plant plasticity in driving ecosystem functioning than previously assumed.4. Synthesis. Our results suggest that plant litter quality varies even at the microscale with heterogeneity in soil resource availability, thereby potentially feeding back on soil properties and sustaining microscale soil fertility patchiness. In parallel, plants of more fertile patches resorbed a greater absolute amount of N, likely benefiting their competitive and reproductive abilities.

Aquaporins: for more than water at the plant–fungus interface?

Aquaporins are abundant membrane proteins that channel water, but also small uncharged molecules, across the membranes of plants, animals and microbes (Maurel et al., 2008). In this issue of New Phytologist, Dietz et al. (pp. 927–940) report on a thorough molecular and functional analysis of the whole aquaporin family of an ectomycorrhizal (ECM) fungus, the basidiomycete Laccaria bicolor. This study is novel as it describes, from an evolutionary perspective, the diversity of aquaporins in a fungus, which, with the exception of the yeast Saccharomyces cerevisiae, had not been extensively studied at a genome scale in these organisms. This study is also of special importance as it addresses the role of aquaporins in both nutrient and water exchanges during symbiosis between plants and fungi, with a potentially great impact on water and nutrient economy in forest ecosystems. In brief, Dietz et al. screened the full genomic sequence of Laccaria bicolor to identify seven open reading frames (ORFs) encoding aquaporin homologues. They used heterologous expression in yeast and Xenopus oocytes to characterize the ability of these homologues to transport water but also various solutes such as glycerol, urea andammonium (NH4+)/ammonia (NH3). It is of note that nitrogen (N) compounds, some of which are putative aquaporin substrates, are, besides water, crucial during ECM symbiosis. Similar to observations of aquaporins in other organisms, various selectivity profiles were identified for the L. bicolor aquaporins; some isoforms specifically transported water whereas others were also permeable to urea, glycerol and/or NH3.

Effects of cattle and manure management on the nutrient economy of mixed farms in East Africa: A scenario study

This paper explores effects of animal and manure management in a dairy unit on the nutrient economy of crop-livestock farms in East Africa. For this purpose, 8 cattle management scenarios have been developed based on farming systems in Mbeere, Kenya (extensive), Wakiso, Uganda (semi-intensive) and Kibichoi, Kenya (intensive). Three baseline scenarios represent present-day cattle management; five improved scenarios use the same dairy breeds but have improved nutrition, using younger grass, more legumes and moderate amounts of concentrates. These improvements strongly increase milk production per cow, but also N, P and K excretion in manure. The 8 cattle management scenarios are combined with 2 levels of manure management technology: a baseline technology, reflecting actual manure management and related losses of plant nutrients, and an improved technology with lower losses. Nutrient losses for each technology level have been derived from a thorough analysis of published information. This showed that current systems of collection and storage of the excreta of confined dairy cows are associated with large nutrient losses, in particular of N. These losses cause serious deficits on the N, P and K balances of the crop-livestock farms. Therefore, significant external N, P and K inputs and better manure management are required to sustain the production levels assumed and to avoid further soil fertility depletion in the region. The paper identifies several possibilities for this and concludes that there is a strong need for integral on-farm studies aiming at development of sustainable dairy production systems.

Uptake and use efficiency of N, P, K, Ca and Al by Al-sensitive and Al-tolerant cultivars of wheat under a wide range of soil Al concentrations

The impacts of acidic soils and Al toxicity on wheat nutrient economy have been scarcely researched under field conditions even though these soils are widely spread in wheat production areas around the world. The main objective of this study was to quantitatively evaluate the element (N, P, K, Ca and Al) economy of an Al-sensitive and an Al-tolerant wheat cultivar growing under different soil Al concentrations at field conditions. To reach this objective, two field experiments were conducted in an Andisol in Valdivia (39°47′18″S, 73°14′05″W), Chile. Treatments were a factorial arrangement of: (i) two spring wheat cultivars (Al-sensitive, Domo.INIA and Al-tolerant, Dalcahue.INIA) and (ii) five exchangeable Al levels (0–2.7 cmol (+) kg −1) with three replicates. At harvest, plant biomass was sampled and divided into 5 organ categories: ears, grains, blade leaves, stems plus sheath leaves and roots. The element content (N, P, K, Ca and Al) in each organ was measured to quantify element uptake and concentration, nutrient uptake efficiency (UPE) and nutrient utilization efficiency (UTE). Element uptake (N, P, K, Ca, and Al) was negatively affected by the increased soil Al concentration in above-ground and root biomass in both cultivars ( R 2 = 0.61–0.98, p < 0.01), although clear differences were found between cultivars. On the contrary, the impact of soil exchangeable Al on the plant element concentration was minor, showing weak associations with soil Al levels. However, the Al concentration in above-ground tissues of the Al-sensitive cultivar was an exception because it increased exponentially with the Al soil concentration ( R 2 = 0.96–0.99, p < 0.001). Nutrient uptake efficiencies, UPEs (N, P, K and Ca), were negatively affected by soil Al concentrations and were well described by linear equations in both cultivars ( R 2 = 0.58–0.98, p < 0.05), with notable differences between them. Both nutrient uptake (capture) and UPE were the traits that best explained above-ground biomass production ( R 2 = 0.82–0.99, p < 0.001, n = 20). Nutrient utilization efficiency, UTEs (N, P, K and Ca) responded more conservatively to the soil Al concentration, except for the Al sensitive cultivar under very high soil Al levels.

Inter-specific Variation in Foliar Nutrients and Resorption of Nine Canopy-tree Species in a Secondary Neotropical Rain Forest

The influence of environmental gradients on the foliar nutrient economy of forests has been well documented; however, we have little understanding of what drives variability among individuals within a single forest stand, especially tropical forests. We evaluated inter- and intra-specific variation in nutrient resorption, foliar nutrient concentrations and physical leaf traits of nine canopy tree species within a 1-ha secondary tropical rain forest in northeastern Costa Rica. Both nitrogen (N) and phosphorus (P) resorption efficiency (RE) and proficiency of the nine tree species varied significantly among species, but not within. Both N and P RE were significantly negatively related to leaf specific strength. Green leaf N and P concentrations were strongly negatively related to leaf mass per area, and senesced leaf nutrient concentrations were significantly positively related to green leaf nutrient concentrations. This study reveals a strong influence of physical leaf traits on foliar nutrient and resorption traits of co-occurring species in a secondary wet tropical forest stand.

Coordinated variation in leaf and root traits across multiple spatial scales in Chinese semi‐arid and arid ecosystems

• Variation in plant functional traits is the product of evolutionary and environmental drivers operating at different scales. Little is known about whether, or how, this variation is coordinated between aboveground and belowground organs across and within spatial scales. • We address these questions using a hierarchically designed dataset of pairwise leaf and root traits related to carbon and nutrient economy of 64 species belonging to 14 plant communities in northern Chinese semi-arid and arid regions. • While both root and leaf traits showed most of their variance among (individuals and) species within communities, leaf trait variance tended to be relatively higher at coarser spatial scales than root trait variance. While leaf nitrogen (N) per area to root N per length ratio increased and specific leaf area to specific root length and leaf [N] to root [N] ratios decreased from semi-arid to arid environments owing to climatic/edaphic shifts, the matching pairs showed a strong pattern of positive correlation that was upheld across spatial scales and geographic areas. • Thus, trade-offs in plant resource investment across organs within individual vascular plants are constrained within a rather narrow range of variation. A new challenge will be to test whether and how such trait coordination is also seen within and across other biomes of the world.

Isotopic analysis of cyanobacterial nitrogen fixation associated with subarctic lichen and bryophyte species

Dinitrogen fixation by cyanobacteria is of particular importance for the nutrient economy of cold biomes, constituting the main pathway for new N supplies to tundra ecosystems. It is prevalent in cyanobacterial colonies on bryophytes and in obligate associations within cyanolichens. Recent studies, applying interspecific variation in plant functional traits to upscale species effects on ecosystems, have all but neglected cryptogams and their association with cyanobacteria. Here we looked for species-specific patterns that determine cryptogam-mediated rates of N2 fixation in the Subarctic. We hypothesised a contrast in N2 fixation rates (1) between the structurally and physiologically different lichens and bryophytes, and (2) within bryophytes based on their respective plant functional types. Throughout the survey we supplied 15N-labelled N2 gas to quantify fixation rates for monospecific moss, liverwort and lichen turfs. We sampled fifteen species in a design that captures spatial and temporal variations during the growing season in Abisko region, Sweden. We measured N2 fixation potential of each turf in a common environment and in its field sampling site, in order to embrace both comparativeness and realism. Cyanolichens and bryophytes differed significantly in their cyanobacterial N2 fixation capacity, which was not driven by microhabitat characteristics, but rather by morphology and physiology. Cyanolichens were much more prominent fixers than bryophytes per unit dry weight, but not per unit area due to their low specific thallus weight. Mosses did not exhibit consistent differences in N2 fixation rates across species and functional types. Liverworts did not fix detectable amounts of N2. Despite the very high rates of N2 fixation associated with cyanolichens, large cover of mosses per unit area at the landscape scale compensates for their lower fixation rates, thereby probably making them the primary regional atmospheric nitrogen sink.

Substantial nutrient resorption from leaves, stems and roots in a subarctic flora: what is the link with other resource economics traits?

*Nutrient resorption and leaching resistance, through their roles in reducing nutrient losses, are important determinants of plant nutrient economy. However, the contributions of fine-stem and fine-root resorption, as well as leaf leaching resistance, have largely been overlooked. *We quantified the relative contributions of these processes to nutrient depletion of these organs during their senescence using 40 subarctic vascular species from aquatic, riparian and terrestrial environments. We hypothesized that interspecific variation in organ nutrient resorption and leaf leaching would be linked to the species' nutrient acquisitive-conservative strategies, as quantified for a set of common-organ nutrient/carbon economics traits. *The subarctic flora generally had both high resistance to leaching and high internal nutrient recycling. Average nutrient resorption efficiencies were substantial for leaves (nitrogen (N), 66 +/- 3% SE; phosphorus (P), 63 +/- 4%), fine stems (N, 48 +/- 4%; P, 56 +/- 4%) and fine roots (N, 27 +/- 7%; P, 57 +/- 6%). The link between nutrient resorption and other nutrient/carbon economics traits was very weak across species, for all three organs. *These results emphasize the potential importance of resorption processes for the plant nutrient budget. They also highlight the idiosyncrasies of the relationship between resorption processes and plant economics, which is potentially influenced by several plant physiological and structural adaptations to environmental factors other than nutrient stress.

Nitrogen availability, leaf life span and nitrogen conservation mechanisms in leaves of tropical trees

Evergreen species of temperate regions are dominant in low-nutrient soils. This feature is attributed to more efficient mechanisms of nutrient economy. Nevertheless, the cashew (Anacardium occidentale- Anacardiaceae), a deciduous species, is native to regions in Brazil with sandy soil, whilst the annatto (Bixa orellana- Bixaceae), classified as an evergreen species native to tropical America, grows spontaneously in regions with more humid soils. Evergreens contain robust leaves that can resist adverse conditions for longer. The physical aspects of the leaves and mechanisms of nutrient economy between the two species were compared, in order to verify whether the deciduous species had more efficient mechanisms that might explain its occurrence in regions of low soil fertility. The mechanisms of nitrogen economy were also compared for the two species at available concentrations of this nutrient. The following were analysed: (i) leaf life span, (ii) physical leaf characteristics (leaf mass per area, and rupture strain), (iii) nitrogenous compounds (nitrogen, chlorophyll, and protein), (iv) nitrogen conservation mechanisms (nitrogen resorption efficiency, resorption proficiency, and use efficiency), and (v) nitrogen conservation mechanisms under different availability of this mineral. The higher values of leaf mass per area and leaf rupture strain found in A. occidentale were related to its longer leaf life span. A. occidentale showed lower concentrations of nitrogen and protein in the leaves than B. orellana. Under lower nitrogen availability, A. occidentale had higher nitrogen resorption proficiency, nitrogen use efficiency and leaf life span than B. orellana. These characteristics may contribute to the adaptation of this species to sandy soils with low nitrogen content.

Water Supply Changes N and P Conservation in a Perennial Grass Leymus chinensis

Changes in precipitation can influence soil water and nutrient availability, and thus affect plant nutrient conservation strategies. Better understanding of how nutrient conservation changes with variations in water availability is crucial for predicting the potential influence of global climate change on plant nutrient-use strategy. Here, green-leaf nitrogen (N) and phosphorus (P) concentrations, N- and P-resorption proficiency (the terminal N and P concentration in senescent leaves, NRP and PRP, respectively), and N- and P-resorption efficiency (the proportional N and P withdrawn from senescent leaves prior to abscission, NRE and PRE, respectively) of Leymus chinensis (Trin.) Tzvel., a typical perennial grass species in northern China, were examined along a water supply gradient to explore how plant nutrient conservation responds to water change. Increasing water supply at low levels (< 9000 mL/year) increased NRP, PRP and PRE, but decreased green-leaf N concentration. It did not significantly affect green-leaf P concentration or NRE. By contrast, all N and P conservation indicators were not significantly influenced at high water supply levels (> 9000 mL/year). These results indicated that changes in water availability at low levels could affect leaf-level nutrient characteristics, especially for the species in semiarid ecosystems. Therefore, global changes in precipitation may pose effects on plant nutrient economy, and thus on nutrient cycling in the plant-soil systems.

Tree mixture effects on aboveground nutrient pools of trees in an experimental plantation in Panama

Mixed plantations might contribute to sus- tainable land use because of complementary nutrient use. Our objective was to assess the nutrient economy of five native tree species and their response to different neighbor trees in an experimental plantation in Panama. In our study, H. crepitans was the least nutrient efficient tree species. H. crepitans produced less biomass in mixtures than in monocultures while Cedrela odorata - the most nutrient efficient species - produced more biomass independent of stem growth rates because they acquired more nutrients in mixtures than in monocultures.Three-species mixtures had in- creased mean Ca concentrations in branches and stems and increased N, P, K, Ca, and Mg storage in aboveground biomass compared to monocultures sug- gesting complementary resource uptake. Our results highlight the need to properly consider species-specific nutrient requirements and species interactions when designing tree mixtures for afforestation.

Mineral nutrient relations in the aquatic carnivorous plant Utricularia australis and its investment in carnivory

Aquatic rootless carnivorous plants usually grow in nutrient-poor waters and take up all nutrients through their shoots, from either water or prey. The carnivorous plant Utricularia australis was sampled from 30 locations in the Třeboň basin, Czech Republic, with the aim of investigating the plant's mineral nutrient economy in relation to its carnivorous habit. Relationships were sought between, firstly, mineral nutrient levels in the ambient water together with prey quantity captured in traps and, N, P, K, Na, Ca, and Mg contents of shoot tissues, and, secondly, the proportion of total plant biomass attributable to traps (i.e., investment in camivory). Even at very oligotrophic sites with low prey capture rates, shoot N and P content was always well above the level below which growth limitation could occur. Plants recycled at least 57 % of their N and as much as 81 % of their P from senescent shoots, although they lost all of the K, Na, Ca, and Mg from senescing tissues. The P and K content of traps was much greater than that in leaves. Regression analyses revealed much greater uptake of N, P, and K from prey than from the ambient water. The proportion of total biomass invested in traps (range 23-61 %) was positively correlated with CO 2 concentration, but negatively with shoot N content. It is suggested that shoot N content acts as a key endogenous factor regulating investment in trap biomass through a negative feedback mechanism. Any decline in shoot N content, for whatever reason, stimulates an increase in trap production, resulting in enhanced prey catching and, concomitantly, an increase in shoot N content. A reduction in trap proportion then follows and the cycle starts again.

Mineral nutrient economy in competing species of Sphagnum mosses

AbstractBog vegetation, which is dominated by Sphagnum mosses, depends exclusively on aerial deposition of mineral nutrients. We studied how the main mineral nutrients are distributed between intracellular and extracellular exchangeable fractions and along the vertical physiological gradient of shoot age in seven Sphagnum species occupying contrasting bog microhabitats. While the Sphagnum exchangeable cation content decreased generally in the order Ca2+ ≥ K+, Na+, Mg2+ &gt; Al3+ &gt; NH4+, intracellular element content decreased in the order N &gt; K &gt; Na, Mg, P, Ca, Al. Calcium occurred mainly in the exchangeable form while Mg, Na and particularly K, Al and N occurred inside cells. Hummock species with a higher cation exchange capacity (CEC) accumulated more exchangeable Ca2+, while the hollow species with a lower CEC accumulated more exchangeable Na+, particularly in dead shoot segments. Intracellular N and P, but not metallic elements, were consistently lower in dead shoot segments, indicating the possibility of N and P reutilization from senescing segments. The greatest variation in tissue nutrient content and distribution was between species from contrasting microhabitats. The greatest variation within microhabitats was between the dissimilar species S. angustifolium and S. magellanicum. The latter species had the intracellular N content about 40% lower than other species, including even this species when grown alone. This indicates unequal competition for N, which can lead to outcompeting of S. magellanicum from mixed patches. We assume that efficient cation exchange enables Sphagnum vegetation to retain immediately the cationic nutrients from rainwater. This may represent an important mechanism of temporal extension of mineral nutrient availability to subsequent slow intracellular nutrient uptake.

The role of arbuscular mycorrhizal colonization in the carbon and nutrient economy of the tripartite symbiosis with nodulated Phaseolus vulgaris

In the tripartite symbiosis between nodulated legume roots and arbuscular mycorrhizal (AM) fungi, symbiont sink strength may depend upon developmental stage and the nutrient benefits to the host plant. The cost-benefits of the tripartite symbiosis were investigated in terms of C-economy and nutrition. Nodulated Phaseolus vulgaris seedlings, with and without AM, were hydroponically grown under high (2 mM) and low (1 μM) P conditions in an N-free Long Ashton nutrient solution. Plants were sequentially harvested at 17, 24 and 31 days after emergence. At each harvest, measurements for biomass, N 2-fixation, photosynthesis, root respiration, calculated C and nutritional economy were taken. Nodular growth was suppressed by the early development of AM colonization. This coincided with higher photosynthetic and respiratory rates in AM plants. These effects were most pronounced under low P when AM colonization peaked. Once AM levels reached the plateau phase, the efficiency of P nutrition increased. This was followed by improved nodular and host growth and enhanced N 2-fixation. This indicates that the AM was the dominant symbiont for host C in the tripartite symbiosis, due to its rapid development and subsequent role in supplying P more effectively to both host and nodules.